1. Technical Field
This invention relates generally to antennas for communication devices, and more particularly to a low profile, multi-band antenna suitable for internal use within a communication device.
2. Background Art
Electronic devices are continually evolving. For example, at one time a mobile telephone was a relatively large device with a long, floppy, protruding antenna. Due to advances in technology, modem mobile telephones are slimmer and lighter. As mobile telephones have gotten smaller in size, so too have the antennas they employ. Antenna design has advanced to the point that some modem mobile telephones do not include protruding antennas at all. They rather rely upon internal antenna structures for communication with cellular towers and base stations. The use of internal antennas has allowed designers and engineers to create sleeker and more fashionable products.
One popular antenna in use today is the planar inverted-F antenna (PIFA). This antenna is widely available and well suited to dual-band operation. “Dual-band” means that the antenna has two resonance frequency bands, and is suitable for communicating in two primary bandwidths. For example, a dual-band planar inverted-F antenna may be used in a dual-band GSM phone operating in both GSM 900 (880 MHz-960 MHz) and GSM 1800 (1710 MHz-1880 MHz) bands. The dual-band planar inverted-F antenna splits in two branches, where the longer branch resonates (thereby producing electromagnetic radiation) in one band, while the shorter branch resonates in another band.
The problems with this type of antenna are two fold: First, they are difficult to design for tri-band operation. For example, a phone required to operate in GSM 900, GSM 1800, and UMTS (1920 MHz-2170 MHz) would not function well in every bandwith, especially given the typical size and volume limitations of modem mobile telephones, if the phone employed a planar inverted-F antenna.
Second, the different branches of the planar inverted-F antenna essentially compete with each other to claim a portion of a given available physical volume in the mobile telephone. The effect of this competition is that each resonant mode has associated therewith a higher Q than it would have if the whole physical volume was provided to each branch. This means that each resonant band becomes narrower, and thus less effective. Thus, there is a limit to the amount the planar inverted-F antenna structure may be reduced in size without affecting performance. In short, to function properly, dual-band planar inverted-F antennas are relatively large. This is a problem for designers who continually want to make mobile communication devices smaller and thinner.
There is thus a need for an improved antenna that functions in multiple bandwidths, yet is more compact in size, which achieves suitable radiated efficiency levels.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.